The Prince Is A Girl: The Beast King's Captive Mate Manga

In the Dark Age, human kingdoms had to offer women as tributes to please the Urekai. Princess Emeriel disguised herself as a prince from childhood to survive. But to save her sister, she willingly became a slave to the Urekai, only to discover she was a rare "Siren"—a woman capable of mating with a beast. Chosen by the High King Beast, who has been berserk for five hundred years, she struggles to survive amidst fear and desire. Secrets, estrus, and soul bonds will push her towards the abyss of fate.